Auxiliary power units are mounted in a chamber within the aircraft body for supplying electricity to auxiliary aircraft components. Preferably, such auxiliary power unit is operating only intermittently. Separate air inlet openings are conventionally provided for introducing useful or fresh air into the aircraft and for supplying cooling air to the auxiliary power unit. These inlet openings or air intakes pass through the skin of the aircraft body. Separate flaps are movably secured to the aircraft body for closing the air inlet opening for the airflow of the useful or fresh air into the aircraft body and for closing the air inlet opening for the cooling airflow to the auxiliary power unit. The separate flaps are controlled in such a manner that both air inlet openings are open when the auxiliary power unit is operating and so that both openings are closed when the auxiliary power unit is not operating.
German Patent Publication DE 34 07 137 corresponding to U.S. Pat. No. 4,655,413 (Genasler et al.) discloses an air intake as described above for an auxiliary power unit, see particularly FIGS. 8 and 9. An auxiliary power unit is, for example, installed in the tail end of a commercial aircraft. Normally, such a unit operates intermittently. Therefore, the air inlets or air intakes are closeable by flaps which in their closed condition are flush with the outer skin of the aircraft body. In order to improve the flow conditions along the outer skin of the aircraft body a boundary layer deflector is provided in the flow direction upstream of the air inlet. Such boundary layer deflector is constructed and mounted to be movable relative to the aircraft body. The boundary layer deflector is positioned or operated in such a manner in response to the respective position of the air intake flap or flaps that when the air intake is open the boundary layer deflector is positioned outside of the aircraft body in its operating position. On the other hand, when the air intake is closed the boundary layer deflector is positioned within the outer contour of the aircraft body.
The above described conventional air intake construction leaves room for improvement particularly with regard to simplifying the construction and reducing the number of components required for performing the above outlined task.